The invention relates to the sphere of rolled or extruded products such as sheets, profiles, wires or tubes made of AlMgMn-type aluminium alloy containing more than 3% by weight of Mg, intended for welded constructions having a high yield stress, good resistance to fatigue and good toughness for structural applications such as ships, industrial vehicles or welded bicycle frames.
The optimum dimensioning of welded structures made of aluminium alloy leads to the use of 5,000 series AlMg alloys according to the Aluminium Association nomenclature, in the cold-worked temper (temper H1 according to the standard NF-EN-515) or partially softened temper (temper H2), or stabilized temper (temper H3), while maintaining high resistance to corrosion (temper H116) rather than the annealed temper (temper O). However, the improvement in the mechanical characteristics relative to the temper O does not usually remain after welding, and certifying and monitoring organizations generally recommend that only the characteristics in temper O be taken into consideration for welded structures. The resistance to fatigue and the fissure propagation rate should also be taken into consideration for dimensioning.
In this sphere, research has concentrated mainly on the implementation of the welding operation itself. There have also been attempts to improve the corrosion resistance of the article by appropriate thermomechanical treatments.
Japanese patent application JP 06-212373 proposes the use of an alloy containing 1.0 to 2.0% of Mn, 3.0 to 6.0% of Mg and less than 0.15% of iron to minimize the reduction in the mechanical strength due to welding. However, the use of an alloy having such a high manganese content leads to a reduction in the resistance to fatigue and in the toughness.
The object of the invention is significantly to improve the mechanical strength and fatigue resistance of welded structures made of AlMgMn alloy, under predetermined welding conditions, without unfavourable consequences for other parameters such as toughness, corrosion resistance and cutting deformation, due to internal stresses.
The invention relates to products for welded constructions made of AlMgMn aluminium alloy composed of (% by weight):
3.0 less than Mg less than 5.0
0.5 less than Mn less than 1.0
Fe less than 0.25
Si less than 0.25
Zn less than 0.40
optionally one or more of the elements Cr, Cu, Ti, Zr such that:
Cr less than 0.25
Cu less than 0.20
Ti less than 0.20
Zr less than 0.20
other elements  less than 0.05 each and  less than 0.15 in total, wherein Mn+2Zn greater than 0.75.
Contrary to earlier research which concentrated on the welding process and the thermomechanical treatments, the inventors have found a particular, range of composition for minor alloying elements, in particular iron, manganese and zinc, leading to an interesting set of properties combining static mechanical characteristics, toughness, resistance to fatigue, resistance to corrosion and cutting deformation, this set of properties being particularly well adapted to the use of these alloys for naval construction, utility vehicles or the welded frames of bicycles.
This set of properties is obtained by combining a low iron content,  less than 0.25%, preferably  less than 0.20%, and even 0.15%, and a manganese and zinc content such that Mn+2Zn greater than 0.75%, preferably  greater than 0.8%. The Mn content should be  greater than 0.5%, preferably  greater than 0.8%, to have adequate mechanical characteristics, but should not exceed 1% if a deterioration in toughness and fatigue resistance are to be avoided. The addition of zinc combined with manganese has been found to have a beneficial effect on the mechanical characteristics of welded sheets and joints. However, it is better not to exceed 0.4% because problems can then be encountered in welding.
The magnesium is preferably kept  greater than 4.3%, because it has a favourable effect on the yield stress and fatigue resistance, but beyond 5% the corrosion resistance is less good. The addition of Cu and Cr are also favourable to the yield stress, but Cr is preferably kept  less than 0.15% to maintain good resistance to fatigue.
The mechanical strength of the sheets depends both on the magnesium content in solid solution and on the manganese dispersoids. It has been found that the volumetric fraction of these dispersoids, which is linked to the iron and manganese contents, should preferably be kept above 1.2%. This volumetric fraction is calculated from the average of the surface fractions measured on polished cuts produced in three directions (length, width and thickness) by scanning electron microscopy and image analysis.
The products according to the invention can be rolled or extruded products such as hot- or cold-rolled sheets, wires, profiles or extruded and optionally drawn tubes.
The sheets according to the invention, which are assembled by butt welding by a MIG or TIG process and with a bevel of the order of 45xc2x0 over about ⅔ of the thickness have, in the welded region, a yield stress R0.2 which can be at least 25 MPa higher than that of a conventional alloy having the same magnesium content, that is a gain of about 20%.
The width of the thermally affected region is reduced by about one third relative to a conventional 5083 alloy, and the hardness of the welded joint increases from about 75 Hv to more than 80 Hv. The welded joints also have a tensile strength exceeding the minimum imposed by organizations monitoring unwelded cold-worked crude sheets.
The sheets according to the invention have fatigue resistance, measured by plane bending with a stress ratio wherein R=0.1 on samples taken in the cross-longitudinal direction, higher than:
105 cycles with a maximum stress  greater than 280 MPa
106 cycles with a maximum stress  greater than 220 MPa
107 cycles with a maximum stress  greater than 200 MPa.
The fissure propagation rate xcex94K, measured when R=0.1, is  greater than 22 Mpa{square root over (m)} when da/dN=5xc3x9710xe2x88x924 mm/cycle and  greater than 26 Mpa{square root over (m)} when da/dN=10xe2x88x923 mm/cycle.
The sheets according to the invention usually have a thickness greater than 1.5 mm. With thicknesses greater than 2.5 mm they can be obtained directly by hot rolling, without the need for subsequent cold rolling and, furthermore, these hot-rolled sheets are less distorted on cutting than cold-rolled sheets.
The products according to the invention have corrosion resistance which is as good as that of normal alloys having the same magnesium content, for example 5083 of common composition, widely used in naval construction.